The present invention relates to a driving apparatus for telescopic antenna equipped to automotive vehicle or the like and to a mechanism for preventing an overload and being applicable to the driving apparatus.
One of overload prevention clutches for the driving apparatus, is provided with driving and driven clutch plates each of which is urged by spring means, and which are formed with rugged surfaces on opposing contact surfaces thereof to transmit the driving force of an electric motor from the driving end to the driven end therethrough, wherein the rugged surface of the driving clutch plate is run on the rugged surface of the driven clutch plate while the driving clutch plate is rotated relative to the driven clutch plate in an over load state.
In another of the overload prevention clutches, the driving and driven clutch plates have flat surfaces which are slidably contacted with each other under a spring-urged state to transmit the driving force generated from the electric motor from the driving end to driven end by means of friction therebetween, wherein the driving clutch plate is slipped onto the driven clutch plate to prevent the electric motor from the overloading.
In accordance with the first overload prevention clutch in which both clutch plates are formed with the rugged portions on the opposing contact surfaces thereof, since the rugged surface of the driving clutch plate is run on the rugged surface of the driven clutch plate in overload state of the clutch as described above and then is restored in a normal state where the driving clutch is meshed with the driven clutch on rugged surfaces thereof, and hereafter the operation is repeated in the clutch until the feeding of electric power to the electric motor is stopped or an overload is eliminated, the clutch generates a discordant sound such as an impact sound. Therefore, the conventional overload prevention clutch does not fit a passenger car in which silent operation is required.
In accordance with the second overload prevention clutch in which the clutch plates are contacted on the plane surfaces thereof to each other, there is no noise generated by the running of the rugged surface of the driving clutch plate on the rugged surface of the driving clutch plate. However, an invasion of water, mud or grease into a gap between the plane surfaces of the two clutch plates makes it difficult for the clutch to transmit a driving force from the driving end to the driven end and there is a fear that the electric motor maybe damaged. In particular, when the overload prevention clutch is applied to a passenger car which is used in adverse circumstances where water, mud or grease or the like violently splashes and temperature and humidity greatly change, it is difficult to avoid the above-described problems in the power transmission.
In addition, since the two clutch plates of each of the conventional overload prevention clutches are directly contacted to each other by means of relatively powerful spring force, a long-term use of that clutch causes the contact surfaces to be worn to degrade transmission capability. Thus, both the overload prevention clutches is poor in durability.